1. Field of the Invention
This invention relates to a plasma-resistant member and also to a plasma-treating apparatus using the same.
2. Description of the Related Art
In a plane-parallel plate-type etching apparatus for etching a silicon oxide film (SiO2) or phosphosilicate glass formed on a body to be processed, an etchant gas of a fluorine compound, such as CF4, C2F6, CHF3 or the like, is used after excitation with a plasma. The fluorine-containing active chemical species generated from these gases corrode Si-based compounds such as silica glass and silicon carbide. To avoid this, anodic oxidized aluminum or alumina ceramics have been used for constituting parts of the etching apparatus used for this purpose. In a step such as of microfabrication or in case where insulating properties are required, more reliable alumina ceramics have been in frequent use.
Alumina ceramics, which are excellent in corrosion resistance against the fluorine plasma used in such an etching apparatus as mentioned above, have been proposed in Japanese Laid-open patent Application Nos. Hei 8-81258 and 8-231266. As the scaling down of a semiconductor integrated circuit is in progress, it is necessary to perform more microfabrication in a higher yield. To meet the necessity, there have been proposed, in place of the parallel-plane type etching apparatus, a variety of low pressure high density plasma. In an apparatus using a microwave as a plasma exciting source, alumina used as a microwave transmitting material has been proposed in Japanese Laid-open Patent Nos. Hei 5-217946 and 6-345527. In these proposals relating to alumina ceramic parts for semiconductor processing apparatus, the purity and grain size of alumina are mainly defined in order to develop a corrosion resistance against the fluorine plasma.
However, as semiconductor devices are scaled down in recent years, there is often required anisotropic etching of a high aspect ratio in an etching process. To meet this requirement, there has now been adopted a method of forming a side wall protecting film so as to suppress the undercut to a minimum and process a deep hole with a small-sized opening.
FIG. 2 principally shows this state as enlarged. In order to form a protecting film on the side walls of an etched deep groove as shown in FIG. 2, a polymer film is deposited on the side wall by use of a chlorine-containing gas such as CClF3, or a carbon-fluorine-based (Cxe2x80x94F based) gas such as C3F8 or C4F8. The term xe2x80x9cpolymerxe2x80x9d used herein means a fluorocarbon polymer or a fluorocarbon polymer containing alumina fine particles and/or fluorinated alumina product.
When an etching treatment is carried out using such a gas as mentioned above, the polymer of the decomposition product of a reactant gas is deposited on the electrode surface of a reaction chamber, the inner walls of the reaction chamber, a clamp ring, an electrode insulating member and the like, thereby forming a film thereon. When this film gradually becomes thick, it is falls off and is re-deposited on the surface of the semiconductor wafer, thereby lowering its yield. To avoid this, the apparatus (reaction chamber) has to be opened every given period of time in the etching operation and cleaned by removal of the polymer therefrom. This cleaning cycle has been so short as to cause the productivity to be lowered.
The measure against the formation of a polymer in the reaction chamber using the alumina parts has been proposed in Japanese Laid-open Patent Application No. Sho 61-289634. In this application, it is stated to find that the polymer is more unlikely to be formed on an aluminum material in comparison with anodic oxidized aluminum. It is also stated that when using the material, the effect of suppressing the formation of the polymer is obtained even though there is used, aside from C2F6 and CHF3, a mixed gas, such as C3F8 and CHF3, C2F6 and C2H4F2 or the like, as an etchant gas.
However, when we made extensive studies using the parallel-plane-type etching apparatus, it was confirmed that a substantial amount of the polymer was formed on the surface of an alumina ceramic part. From this, it is assumed that the results set out in the Japanese Laid-open Patent Application No. Sho 61-289634 are obtained only under limited conditions.
Japanese Laid-open Patent Application No. Hei 10-32237 proposes a clamp ring, which is made of a porous alumina sintered product having an average grain size of 20 xcexcm or over. This clamp ring has a greater effect of suppressing a once deposited polymer from falling off than a conventional one. However, this ring is porous, so that there arises the problem that alumina particles may fall off or the ring may be broken owing to its insufficient strength when suffering ionic impact or other mechanical shock at the time of etching.
Especially, the insufficiency of strength presents another problem on the clamp ring when urged against a lower electrode for fixing a body to be processed therewith, or on an electrode insulating member when attached with screws. Moreover, the polymer is deposited in large amounts on a clamp ring, an electrode insulating member, a focus ring, a covering body or the like, located closely to the electrodes, and thus, the falling off or breakaway of the polymer presents a more serious problem.
An object of the invention is to provide a plasma-resistant member, which does not involve any breakaway of alumina particles and is excellent in mechanical strength, and is able to suppress the breakaway of a once deposited polymer, and also a plasma treating apparatus using the member.
Another object of the invention is to provide a plasma-resistant member, which enables one to prolong an apparatus-cleaning cycle for removal of a deposited polymer, and a plasma-treating apparatus using the member.
The plasma-resistant member of the invention should preferably be used in a reaction chamber of a plasma-treating apparatus. The plasma-resistant member is made of a dense alumina sintered product having an average grain size of 18-45 xcexcm, a surface roughness Ra of 0.8-3.0 xcexcm, and a bulk density of 3.90 g/cm3 or over. It is preferred that the dense alumina sintered product has a purity of not less than 99.8%, an Si content of 200 ppm or below, an alkali metal content of 100 ppm or below.
The plasma treating apparatus comprises an electrode insulating member for electric insulation between at least one of an upper electrode and a lower electrode and a reaction chamber, a clamp ring urging a peripheral portion of a treating surface of a body to be treated against the lower electrode to hold the surface thereat, a focus ring provided in the vicinity of the upper electrode or lower electrode for effectively transmitting reactive ions toward the treating surface of the body to be treated, and a covering member provided to cover the inner walls of the reaction chamber therewith. At least one of the electrode insulating member, the clamp ring, the focus ring and the covering member should be constituted of such a plasma-resistant member.
The plasma treating apparatus may also be arranged to comprise an electrode insulating member for electric insulation between an upper electrode and a reaction chamber, an electrostatic chuck for electrostatically attracting and holding a body to be treated by application of a high voltage to an electric conductor member thereof, a focus ring provided in the vicinity of the upper electrode or lower electrode for effectively transmitting reactive ions toward the treating surface of a body to be treated, and a covering member for covering the inner walls of the reaction chamber. In this arrangement, at least one of the electrode insulating member, the electrostatic chuck, the focus ring and the covering member should be constituted of the plasma-resistant member.
Further, the plasma treating apparatus may be arranged to comprise an electrode insulating member for electric insulation between at least one of an upper electrode and a lower electrode and a reaction chamber, an electrostatic chuck for electrostatically attracting and holding a body to be treated by application of a high voltage to an electric conductor member thereof, a focus ring provided in the vicinity of the upper electrode or lower electrode for effectively transmitting reactive ions toward the treating surface of a body to be treated, a covering member for covering the inner walls of the reaction chamber, and a cover body for covering a peripheral portion of the treating surface of the body to be treated in a non-contact fashion. In this arrangement, at least one of the electrode insulating member, the electrostatic chuck, the focus ring, the covering member and the cover body should be constituted of the plasma-resistant member.
The present invention relates to an improvement in the plasma-resistant member. Examples of such a member include an electrode insulating member, a focus ring, a clamp ring, an electrostatic chuck, a covering member covering the inner walls of a reaction chamber, and a cover body of the plasma treating apparatus. These are plasma-resistant members, which are made of a dense alumina sintered product and used in the reaction chamber. The average grain size and surface roughness are, respectively, defined in the following manner.
First, the method of measuring the average grain size is described. The measurement was accorded to the method set out at page 7 of xe2x80x9cCharacterization Techniques of Ceramicsxe2x80x9d published by The Corporation of the Society of Ceramics and on Jul. 25, 1987. The method used is a so-called Planimetric technique, which has been reported by Z. Jeffries in xe2x80x9cChem. Met. Engrs., 16, 503-504 (1917); ibid., 18, 185 (1918)xe2x80x9d, and this technique is referred to and described as the measuring method. The method is described below.
A circle whose area (A) is known is drawn on a textural photograph, and the number of grains per unit area is determined from the number of grains nc in the circle and the number of grains ni on the circumference of the circle according to the following equation:
NG={nc+(xc2xd)ni}/(A/m2)
wherein m is a magnification. Since 1NG is an area occupied by one grain, the diameter corresponding to the circle is obtained as 2/(xcfx80NG)1/2.
Next, the method of measuring the surface roughness is described. The roughness was measured according to the method described in JIS B 0601.
Ra means a value, expressed in terms of micrometers (xcexcm), which is obtained according to the following equation when only a reference length is withdrawn from a roughness curve along the direction of a mean line wherein an axis of x is taken along the direction of a parallel line of the withdrawn portion and an axis of y is taken as along the direction of a longitudinal magnification under which a roughness curve is expressed by y=f(x):       R    a    =            1      λ        ⁢          xe2x80x83        ⁢                  ∫        0        λ            ⁢                        "LeftBracketingBar"                      f            ⁢                          xe2x80x83                        ⁢                          (              x              )                                "RightBracketingBar"                ⁢                  xe2x80x83                ⁢                  ⅆ          x                    
wherein xcex is a reference length.
The plasma-resistant member (alumina sintered product) of this invention should preferable have an average grain size of 18-45 xcexcm. When the average grain size of the alumina sintered product is smaller than 18 xcexcm, the polymer film deposited on a member is liable to fall off. In contrast, when the size is 18 xcexcm or over, the deposit is difficult to fall off.
When an alumina sintered product is ground, the alumina particles suffer grain boundary fracture and transgranular fracture. At the time of deposition of the polymer on the portion, it is very likely to be deposited along the sites of both grain boundary fracture and transgranular fracture appearing on the surface of the alumina sintered product. The grain boundary fracture surface is so smooth that the adhesion of the film thereto is small. The transgranular fracture surface is so finely split that the adhesion of the film thereat is great.
If the average grain size of the alumina rintered product is smaller than 18 xcexcm, the ratio of the grain boundary fracture surface to a ground surface becomes large. Accordingly, the adhesion between the fracture surface of the particles and the polymer becomes poor, permitting easy breakaway. On the other hand, when the average grain size of the alumina sintered product is 18 xcexcm or over, the ratio of the transgranular fracture surface is large, with the result that the polymer is in strong adhesion to the fracture surface portion of the particles, thus leading to a difficulty in the breakaway.
The adhesion mentioned above is a force required for separating the film by pulling vertically. The thermal stress caused by thermal shock or the like becomes highest at the interface between the film and the alumina sintered product, and occurs in a direction parallel to the interface. Where the average grain size is as larger as 18 xcexcm or over, the recess formed through the grain boundary fracture becomes great, and the polymer film filled in such a great recess serves as xe2x80x9canchorxe2x80x9d, thereby showing a great resistance to the thermal stress. This is considered to be one of factors as to why the polymer film is difficult to fall off.
In case where the average grain size in a member is so large as to exceed 45 xcexcm, the strength of the member lowers. Moreover, when the grain size exceeds 45 xcexcm, it is difficult to stably produce a dense alumina sintered product. For these reasons, the average grain size of the alumina sintered product is defined in the range of 18-45 xcexcm in the invention.
The surface roughness Ra of the alumina sintered product should be 0.8-3.0 xcexcm. Where the surface roughness Ra is smaller than 0.8 xcexcm, the thermal stress generated in association with a heat cycle cannot be mitigated, resulting in the ready breakaway of the deposit film.
The plasma etching apparatus is operated while repeating discharge and exchange of a semiconductor wafer used as a body to be treated, with a fresh one. In order to protect the resist film of the wafer and a protective film at side walls, the semiconductor wafer is invariably cooled. Accordingly, the members around the semiconductor wafer inside the apparatus undergo repetition of a heat cycle including heating resulting from discharge and the ionic impact accompanied thereby, and the temperature drop caused by the stop of the discharge and the movement and mount of a wafer. In order to mitigate the stress caused by the heat cycle and prevent the breakaway of a polymer deposit film, the lower limit of the surface roughness of the alumina sintered product should preferably be at 0.8 xcexcm as expressed by Ra.
The adhesion of the polymer deposited on a member such as a clamp ring in the course of etching is caused by a dispersion force ascribed mainly to the van der Waals force, and there is no chemical bonding between the deposit film and the member. Accordingly, it is considered that the deposit of the polymer depends greatly on the cleanliness of the surface and its physical properties.
We made extensive studies and, as a result, found the relation between the smoothness of a member and the breakaway of a polymer deposit film. More particularly, it was found that the polymer deposit film was most difficult to fall off when the member was finished to have a surface roughness Ra of 0.8-3 xcexcm and the average grain size of an alumina sintered product was 18 xcexcm or over.
The upper limit of the surface roughness of the alumina sintered product is preferably 3.0 xcexcm as expressed by Ra. When Ra exceeds 3.0 xcexcm, there is the apprehension that alumina particles come off from the irregularity on the surface when the member suffers mechanical shock, along with the alumina sintered product lowering in its mechanical strength. In order to make the surface roughness of the alumina sintered product within the above defined range, finishing can be performed according to ordinary diamond grinding.
If the alumina sintered product is not dense, strength lowers, so that the alumina particles may fall off owing to the ionic impact at the time of etching. Accordingly, the bulk density of the alumina sintered product should preferably be at a level of 3.90 g/cm3 or over.
The alumina sintered product should be one which has a purity as high as 99.8% or over. It is preferred that the amount of Si is at 200 ppm or below, and the amount of an alkali metal is at 100 ppm or below. If the Si exceeds 200 ppm in amount and the alkali metal exceeds 100 ppm in amount, the corrosion resistance to the fluorine species becomes worsened.
These are true of the members (i.e. members on which the polymer is possibly deposited) including the clamp ring 1, focus ring 17, electrode insulating member 10b for insulation between at the electrode and the reaction chamber, and covering member 6 for covering the inner walls of the reaction chamber, which are employed in the reaction chamber of the plasma treating apparatus.